This invention relates to a particular class of non-stoichiometric cerium and terbium co-activated magnesium aluminate phosphors which have been found particularly useful in fluorescent lamps. Already known stoichiometric phosphors of this type have the general formula: EQU Ce.sub.1-x-y La.sub.x Tb.sub.y Mg Al.sub.11 O.sub.19
wherein x is an optional constituent in an amount up to about 0.50, and y is in the range from about 0.20 up to about 0.50,
and with said class of phosphors being described in Dutch patent application No. 7,214,862 and U.S. Pat. No. 3,937,998. In the conventional method of preparing said phosphor materials as further disclosed in U.S. Pat. No. 4,150,321, a mixed batch of ingredients which yields said phosphor material upon firing is prepared having the batch proportions of all metal ions as set forth in the aforementioned general formula and which batch is then fired at a temperature range between about 1550.degree. C. up to 1700.degree. C. in a reducing atmosphsphere, such as hydrogen. The batch mixture according to said prior art method of preparation may further contain solid fluxing agents such as boric acid or a metal borate in order to lower the reaction temperature of phosphor formation but which is also recognized to reduce the phosphor emission as compared to preparation without flux. In both of said method variations, moreover, the phosphor material is produced as a very hard mass requiring extensive milling to produce a free-flowing powder having a small enough particle size for most product applications, such as fluorescent lamps and the like. A second firing is often required after such milling in order to improve phosphor brightness sufficiently for acceptable product use.
A different preparation method for said stoichiometric aluminate phosphor material is disclosed in East German Patent No. 0152575 wherein a gaseous fluxing agent, namely hydrogen chloride, is employed to reduce the firing temperatures to 1200.degree. C.-1350.degree. C. and which produces a phosphor material requiring only minimal grinding to yield a free-flowing powder. According to said patent, the phosphor crystal size was said to depend upon the hydrogen chloride concentration with higher acid concentrations providing larger crystals. Said hydrogen chloride concentration was said to vary between about 0.5% and 100% by volume. On the other hand, phosphors prepared at the specified firing temperatures were found to have lower brightness than provided at higher firing temperatures.
Accordingly, it is a principal objective of the present invention to provide a class of said type phosphor materials exhibiting satisfactory brightness especially for lamp product applications and which can now be prepared in a more advantageous manner than the present commercial method of preparation.
Another important object of the present invention is to prepare a class of said phosphor materials which can be prepared by varying the method of preparation so as to produce desired variation in the phosphor emission characteristics.
Still another important object of the present invention is to prepare a class of said phosphor materials wherein desired variation of the phosphor emission characteristics does not substantially decrease phosphor brightness.
A still further important objective of the present invention is to provide improved fluorescent lamps utilizing the present phosphor materials.